This article describes how to limit users to one active SSL VPN connection at a time on Fortigate Series.
From the GUI to VPN -> SSL VPN Portals, edit SSL-VPN Portal and enable: 'limit users to one SSL-VPN connection at a time'.
Following command can be used too:
config vpn ssl web portal
edit <portal name>
set limit-user-logins en
end
Then ask the user to disconnect and connect again.
It will prompt a message :
'You already have an open ssl vpn connection , opening multiple connections are not permitted.
If you proceed, your other connection will be disconnected'
Select 'Login' anyway.